Google does not take a 30% cut of all content sold through apps on Google Play. On Android, the in-app purchase rule only applies to content that can only be consumed via apps (so not Netflix, Amazon, etc.).

And even on iOS, Amazon still has video apps, they just don't allow you to buy or rent directly within the app: https://itunes.apple.com/app/amazon-video/id545519333

There's nothing stopping Amazon from supporting Apple TV or Chromecast, except itself.